Avoiding thunderstorms on arrival into atl we were cleared by approach to turn right a maximum of 175 degree heading and descend to 10000 ft. We penetrated a cumulu nimbus with moderate turbulence in the turn and descent profile. During this sequence, the autoplt disengaged from cmd (command) to cws (control wheel steering) position. Upon realization that we weren't leveling off at 10000 ft, disengaged the autoplt completely and leveled off at 9600 ft. I started to recover to 10000 and approach notified us of our restriction and then cleared us to 9000 ft. Cause: assume an automatic malfunction. Prevention: a C10R (report) being submitted to the company prevention. No logbook write up at that time because of the confusion of the events. The widebody transport manual states that altitude protection is still protected in cws. The first officer and I, as a habit, verbally confirm on an altitude change that the annunciator panel 'altitude alarm' is illuminated. We both felt strongly that in this instance, it was armed. I surmise that the autoplt dropped to the cws position during the altitude capture mode and this disruption cancelled the level off protection. I can find nothing in the prm addressing this situation. Note: an audio warning of the autoplt dropping from cmd to cws position possibly would alert a flight crew of a potential hazardous situation.

Narrative: AVOIDING TSTMS ON ARR INTO ATL WE WERE CLRED BY APCH TO TURN R A MAX OF 175 DEG HDG AND DSND TO 10000 FT. WE PENETRATED A CUMULU NIMBUS WITH MODERATE TURB IN THE TURN AND DSCNT PROFILE. DURING THIS SEQUENCE, THE AUTOPLT DISENGAGED FROM CMD (COMMAND) TO CWS (CTL WHEEL STEERING) POS. UPON REALIZATION THAT WE WEREN'T LEVELING OFF AT 10000 FT, DISENGAGED THE AUTOPLT COMPLETELY AND LEVELED OFF AT 9600 FT. I STARTED TO RECOVER TO 10000 AND APCH NOTIFIED US OF OUR RESTRICTION AND THEN CLRED US TO 9000 FT. CAUSE: ASSUME AN AUTO MALFUNCTION. PREVENTION: A C10R (RPT) BEING SUBMITTED TO THE COMPANY PREVENTION. NO LOGBOOK WRITE UP AT THAT TIME BECAUSE OF THE CONFUSION OF THE EVENTS. THE WDB MANUAL STATES THAT ALT PROTECTION IS STILL PROTECTED IN CWS. THE FO AND I, AS A HABIT, VERBALLY CONFIRM ON AN ALT CHANGE THAT THE ANNUNCIATOR PANEL 'ALT ALARM' IS ILLUMINATED. WE BOTH FELT STRONGLY THAT IN THIS INSTANCE, IT WAS ARMED. I SURMISE THAT THE AUTOPLT DROPPED TO THE CWS POS DURING THE ALT CAPTURE MODE AND THIS DISRUPTION CANCELLED THE LEVEL OFF PROTECTION. I CAN FIND NOTHING IN THE PRM ADDRESSING THIS SITUATION. NOTE: AN AUDIO WARNING OF THE AUTOPLT DROPPING FROM CMD TO CWS POS POSSIBLY WOULD ALERT A FLC OF A POTENTIAL HAZARDOUS SITUATION.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.